Electrochemical cell electrodes

ABSTRACT

Coal based carbon foams that are produced by the controlled heating of coal particulate in a mold and under a non-oxidizing atmosphere and subsequently graphitized have been found to provide excellent electrode materials for electrochemical cell applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 09/888,977, filed Jun. 25, 2001 now U.S. Pat. No.6,899,970, specifically herein incorporated by reference in itsentirety.

This invention was made with Government support under N0014-00-C-0062awarded by the Office of Naval Research. The Government has certainrights in the invention.

FIELD OF THE INVENTION

The present invention relates to electrodes for fuel cells and moreparticularly to relatively inexpensive carbon foam such electrodesproduced from coal.

BACKGROUND OF THE INVENTION

Diminishing supplies of fossil fuels and growing environmental concernscontinue to drive research for the development of alternative sources ofenergy. Each alternative energy source faces the same barriers of costand efficiency. Fuel cells, based on the conversion of hydrogen fuel andoxygen from the air into electricity, offer unique potential as energysources, especially for transportation applications. Vehicles powered byfuel cells would have essentially unlimited range because they could berefueled quickly and conveniently.

Only recently has there been significant attention directed to thepotential of fuel cells for commercial vehicles. Their efficiency, powerdensity and low emission potential have progressed over the past decadeand they are beginning to show potential for zero-emission vehicles. Thefirst fuel cell bus was completed in 1993 and several smaller fuel cellvehicles are now operating in Europe. Beyond transportation, fuel cellshave been used to generate electricity in spacecraft, large militarynaval vessels and prototype power plants. Significant research isongoing in Canada, Japan, the Netherlands as well as in the U.S. Fuelcells are expected to play an increasingly important role in decreasingfossil fuel dependency and improving air quality.

Fuel cells can operate on a variety of fuels including hydrogen andhydrocarbons such as methanol, ethanol and natural gas. Several fuelscell technologies are being considered, but currently the two mostpromising technologies for vehicular applications appear to be; 1)proton exchange membrane fuel cells, and; 2) phosphoric acid fuel cells.

In such fuel cells, the chemical energy from oxidation of a gaseous fuelis converted directly to electrical energy. Fuel cells differ frombatteries in that reactants are supplied from an external source in fuelcells. The fuel and oxidizing gases are bubbled into separate chambersconnected by a porous disk through which an electrolyte such aspotassium hydroxide (KOH) can pass. Inert electrodes, often comprised ofcarbon, mixed with a catalyst such as platinum, are inserted into bothchambers. When electrical connection is made between the electrodes andoxidation-reduction reaction takes place, forming water at the anode andliberating electrons upon the oxidation of hydrogen. These electronsmigrate to the cathode, where they reduce oxygen.

Gas diffusion electrodes are currently the most important class ofelectrodes used in fuel cells of his type. The morphology andcomposition of the electrode material, the mass transport and electricalresistance characteristics of the material in the three phase region,and the distribution of catalysts and surfactants are all of criticalimportance in such fuel cells. The ability to tailor the electricalconductivity, cell size and connectivity and inertness to alter wettingoffer attractive benefits for the use of tailorable carbon foamcompositions and structures in such fuel cell applications.

A porous electrode that is inexpensive to produce and readily formed,conducts electricity well, promotes mass transfer of electrolyte andmaintains consistent performance over its useful life is the holy grailof fuel cell research. The interest in reducing cost and weight andincreasing the efficiency of the processes occurring in fuel celloperation is placing increasingly difficult demands on materials forelectrode construction.

Fuel cell electrodes are commonly comprised of sintered metals, woven ornon-woven carbon fiber mat or activated carbons. Each of theseapproaches has shortcomings. Firstly, electrodes base on sintered metalsrely on porosity at particle interstices as mass transfer paths. Poorinterstitial connectivity results in tortuous paths and reducedelectrolyte transfer. Secondly, activated carbons and metals also sufferfrom changing performance with time. As these electrode materials absorbelectrolyte or cell products, their efficiency changes. Carbon xerogelsand aerogels are also being considered for electrode usage, but theirdurability in service and their cost pose significant hurdles. Whenconfined to small spaces such as pores of membranes or porous electrodesor ion channels, electrochemical processes proceed quite differentlythan in the bulk state. One example of a phenomenon that can be detectedin such environments is non-neutrality. Confinement reduces the numberof ions in a micropore, and the counter ion concentration is notsufficient to balance the wall charges. Because of capillary and surfaceforces, flow and transport art quite different in this region than thebulk as well.

Accordingly, there still exists a significant requirement for improvedfuel cell electrode materials that do not suffer from the previouslydescribed shortcomings.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a fuelcell electrode materials that is relatively less costly than currentelectrode materials.

It is another object of the present invention to provide a fuel cellelectrode material that demonstrates consistent and reliable performancecharacteristics over its useful life.

It is yet a further object of the present invention to provide a fuelcell electrode that exhibits excellent mass transfer properties whileconcurrently being highly inert and resistant the various chemicalspresent in a typical fuel cell environment.

It is yet another object to provide a fuel cell electrode and materialthat can be custom designed to optimize the performance of a particularfuel cell chemistry at the micropore level.

SUMMARY OF THE INVENTION

Coal based carbon foams having a density of preferably between about 0.1g/cm³ and about 1 g/cm³ that are produced by the controlled heating ofcoal particulate in a mold and under a non-oxidizing atmosphere andsubsequently carbonized and graphitized have been found to provideexcellent electrode materials for fuel cell applications. Such materialscan be easily tailored or custom designed to optimize the performance ofthe chemical systems of a broad range of fuel cell designs with onlyminor modification of the fabrication process.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of showing the general relationship between gasevolution and time/temperature at various operating pressures andtemperatures in the process for the manufacture of carbon foams of thetype found useful in accordance with the present invention.

FIG. 2 is a schematic depiction of a fuel cell of the type in which thecarbon electrodes of the present invention find use.

FIG. 3 is a schematic depiction of the operation of the fuel cell ofFIG. 2.

FIGS. 4 and 5 are scanning electron micrographs of reticulated foam asdescribed herein at 10× and 50× magnification respectively.

FIGS. 6 and 7 are optical micrographs of cellular and irregular cellularstructure respectively at 50× magnification.

FIG. 8 is a graph showing the electrical resistivity of coal-basedcarbon foams heat treated to temperatures between about 500° C. andabout 2500° C.

FIG. 9 is a graph showing the interplanar spacing for graphitizedcoal-based foams.

FIG. 10 is a graph showing stack height for graphitized coal-basedcarbon foams.

FIG. 11 is a graph of coherence length for graphitized coal-based carbonfoams.

FIG. 12 is a graph showing crystallite aspect ratio for graphitizedcoal-based carbon foams.

FIGS. 13 through 17 are graphs showing a variety of mechanicalproperties of graphitized coal-based carbon foams.

FIG. 18 is a cyclic voltammogram of ferricyanide over a glassy carbonelectrode.

FIG. 19 is a schematic cyclic voltammogram in the Ag/AgCl system.

FIGS. 20 through 27 are cyclic voltammograms for a variety of electrodematerials evaluated as described hereinafter.

DETAILED DESCRIPTION

FIG. 2 presents a schematic drawing of a typical fuel cell of the typefor which the electrodes of the present invention can be used. Such afuel cell 10 comprises a solid-state polymer electrolyte membrane 12having a pair of gas diffusion electrodes 14 and 16 facing each other onopposed planar sides of polymer electrolyte membrane 12. Gas diffusionelectrode 14 is designated the fuel electrode or anode while electrode14 is designated the oxidation electrode or cathode.

As shown schematically in FIG. 3, fuel, in the case shown hydrogen (H₂)enters porous anode 14 which may contain an appropriate and well knowncatalyst, activator or intercalator, migrates selectively throughpolymer electrolyte membrane 12 as hydrogen ions (H⁺) and enters porouscathode 16 that may also contain an appropriate catalyst where thehydrogen ions combine with oxygen (O₂) from input air to generateelectrons or electrical energy (e⁻) that flow through conductor 18 topower a suitable electrical device 20, in the case depicted a lightbulb, and thence though conductor 18 to anode 14 forming a closedelectrical circuit. The porous carbon electrodes of the presentinvention, find application as either or both of anode 14 and cathode16.

Tailored carbon foams that do not experience performance changes overtime due to adsorption/reaction by electrolyte or product species, arerelatively inexpensive to produce and highly durable offer a solution tothe numerous and perplexing problems described above. Furthermore, thelow cost of carbon foams precursors and processing helps to reduce someof the initial cost concerns of fuel cell construction. Additionally,foams having very different cell sizes, cell connectivities, densitiesand even surface chemistries can be fabricated with only slight processmodifications. These can be accentuated by the same activation andintercalation techniques as traditionally used to offer almost unlimitedpotential for custom electrode property design. Illustrations of thedesign potential for foams structures described herein are presented inFIGS. 4-7. A very open, reticulated structure is shown in FIGS. 4 and 5.This structure is very similar to that of polymeric or vitreous carbonfoams (largely composed of 10 to 15-sided polyhedral that are bounded bysolid ligaments). With minor adjustments to the precursor and processconditions as described below, however, foams having a less connectedcell structure, such as shown in FIGS. 6 and 7, can be created.

The electrical properties and mechanical properties of a given foamstructure can be tailored through heat treatment. For a material verysimilar to that shown in FIG. 7, electrical resistivity can be decreasedby almost ten orders of magnitude by heat treatment to high temperatures(between about 500 and 2500° C.) after the foaming process, asillustrated in FIG. 8.

Porous, coal-based, carbon foams that can be produced from inexpensivebituminous coal powders by a controlled coking process provide thefoundation for the novel electrodes described herein. As described ingreater detail hereinafter, the coal is first foamed in an autoclave. Itis then be further heat-treated to further establish its mechanical,thermal, and physical properties. “Green” foams, i.e. foams that havebeen foamed but not further heat treated, still contain an appreciablequantity of organic matter (e.g., small aliphatic groups) that wouldruin their electrochemical performance. When calcined at 1000-1200° C.under inert gas to remove these materials, such foams are essentially100% carbon, have high electrical conductivity, compressive strength,impact resistance, and low thermal conductivity. Heat treating at highertemperatures, such as above 1700° C. increases graphitic ordering andresults in increases in electrical conductivity, thermal conductivity,and elastic modulus. Thus, foam properties can be designed through (1)precursor selection (coal-based starting material), (2) foaming processconditions, and (3) heat treatment conditions. Carbon foams produced bythese processes can be machined by conventional methods and require nospecial tooling or conditions. Graded foams, or foams having designeddensity gradients through their thickness, have also been developed.This technique may allow the foam to be further tailored to meetlocalized property requirements.

According to the present invention, a preformed, low density, i.e., fromabout 0.1 to about 0.8 g/cm³, and preferably from about 0.1 to about 0.6g/cm³, cellular carbon produced from powdered coal particulatepreferably less than about 1 mm in diameter by the controlled heating ofthe powdered coal in a “mold” under a non-oxidizing atmosphere andsubsequently carbonized and graphitized is used as the electrodes in afuel cell.

The starting material coal may include bitumen, anthracite, or evenlignite, or blends of these, but are preferably bituminous,agglomerating coals that have been comminuted to an appropriate particlesize, preferably to a fine powder below about −60 to −80 mesh. As usedherein, the term “coal-based” is meant to define that the cellularproducts described herein are prepared or manufactured by the“controlled swelling” of ground or comminuted coal with subsequentcarbonization and graphitization procedures applied as required toobtain the required electrical, physical, etc. properties.

The cellular coal-based carbon foams described herein aresemi-crystalline or more accurately turbostratically-ordered and largelyisotropic i.e., demonstrating physical properties that are approximatelyequal in all directions. These coal-based cellular carbon foamstypically exhibit pore sizes on the order of less than 100μ, althoughpore sizes of up to 2000μ are possible within the operating parametersof the process described. The thermal conductivities of the coal-basedcarbon foams are generally less than about 1.0 W/m/° K. Typically, thecoal-based carbon foams used in accordance with the present inventiondemonstrate compressive strengths on the order of from about 2000 toabout 4000 psi at densities of from about 0.4 to about 0.5 g/cm³.

The production method of the coal-based, carbon foams utilized as fuelcell electrodes as described herein comprises: 1) heating a coalparticulate of preferably small i.e., less than about 1 mm particle sizein a “mold” and under a non-oxidizing atmosphere at a heat up rate offrom about 1 to about 20° C./min. to a temperature of between about 300and about 700° C.; 2) soaking at a temperature of between about 300 and700° C. for from about 10 minutes up to about 12 hours to form a preformor finished product; and 3) controllably cooling the preform or finishedproduct to a temperature below about 100° C. The non-oxidizingatmosphere may be provided by the introduction of inert or non-oxidizinggas into the “mold” at a pressure of from about 0 psi, i.e., freeflowing gas, up to about 500 psi. The inert gas used may be any of thecommonly used inert or non-oxidizing gases such as nitrogen, helium,argon, CO₂, etc.

It is generally not desirable that the reaction chamber be vented orleak during the heating and soaking operation. The pressure of thechamber and the increasing volatile content therein tends to retardfurther volatilization while the cellular product sinters at theindicated elevated temperatures. If the furnace is vented or leaksduring soaking, an insufficient amount of volatile matter may be presentto permit inter-particle sintering of the coal particles thus resultingin the formation of a sintered powder as opposed to the desired cellularproduct. Thus, according to a preferred embodiment of the presentprocess, venting or leakage of non-oxidizing gas and generated volatilesis inhibited consistent with the production of an acceptable carbon foamproduct.

Additional more conventional blowing agents may be added to theparticulate prior to expansion to enhance or otherwise modify thepore-forming operation.

The term “mold”, as used herein is meant to define a mechanism forproviding controlled dimensional forming of the expanding coal. Thus,any chamber into which the coal particulate is deposited prior to orduring heating and which, upon the coal powder attaining the appropriateexpansion temperature, contains and shapes the expanding porous coal tosome predetermined configuration such as: a flat sheet; a curved sheet;a shaped object; a building block; a rod; tube or any other desiredsolid shape can be considered a “mold” for purposes of the instantinvention. Mold materials include glass and ceramics as well as aluminumand steel. As will be explained more completely below, the selection ofmold material and consequently heating/volitization rates can affectcell formation and product properties and is consequently an importantconsideration in the production of the foams used as fuel cellelectrodes.

As will be apparent to the skilled artisan familiar with pressurized gasrelease reactions, as the pressure in the reaction vessel, in this casethe mold, increases from 0 psi to 500 psi, as imposed by thenon-oxidizing gas, the reaction time will increase and the density ofthe carbon foam will increase as the size of the “bubbles” or poresproduced in the expanded coal decreases. Similarly, a low soaktemperature at, for example about 400° C. will result in a larger poreor bubble size and consequently a less dense expanded foam than would beachieved with a soak temperature of about 600° C. Further, the heat-uprate will also affect pore size, a faster heat-up rate resulting in asmaller pore size and consequently a denser expanded coal product than aslow heat-up rate. These phenomenon are, of course, due to the kineticsof the volatile release reactions which are affected, as just described,by the ambient pressure and temperature and the rate at which thattemperature is achieved. These process variables can be used to customproduce carbon foams in a wide variety of controlled densities,strengths etc. These results are graphically represented in FIG. 1 wherethe X axis is gas release, the Y axis is time and the individual curvesrepresent different pressures of inert gas P₁, P₂, and P₃, differentheat-up rates HR₁, HR₂, and HR₃, and P₁<P₂<P₃ and HR₁<HR₂<HR₃.

Cooling of the preform or carbon foam product after soaking is notparticularly critical except as it may result in cracking of the preformor product as the result of the development of undesirable thermalstresses. Cooling rates less than 10° C./min to a temperature of about100° C. are typically used to prevent cracking due to thermal shock.Somewhat higher, but carefully controlled, cooling rates may however, beused to obtain a “sealed skin” on the open cell structure of the productas described below. The rate of cooling below 100° C. is in no waycritical.

After expanding the coal particulate as just described, the carbon foamproduct is an open celled material. Several techniques have beendeveloped for “sealing” the surface of the open celled structure toimprove its adhesive capabilities for further fabrication and assemblyof a number of parts. For example, a layer of a commercially availablegraphitic adhesive can be coated onto the surface and cured at elevatedtemperature or allowed to cure at room temperature to provide anadherent skin. Alternatively, the expansion operation can be modified bycooling the carbon foam or preform rapidly, e.g., at a rate of 10°C./min or faster after expansion. It has been discovered that thisprocess modification results in the formation of a more dense skin onthe carbon which presents a closed pore surface to the outside of thepreform. At these cooling rates, care must be exercised to avoidcracking of the preform.

After expanding, the coal-based carbon foam preform is readilymachineable, sawable and otherwise readily fabricated using conventionalfabrication techniques.

Subsequent to production of the carbon foam as just described, it issubjected to carbonization and/or graphitization according toconventional processes to obtain particular properties desirable forspecific application as a fuel cell electrode. Activation, for example,by ozone or carbon dioxide, may also be performed, if activation of thecoal-based expanded carbon foam product would be useful in certain fuelcell applications. Additionally, a variety of additives and structuralreinforcers may be added to the carbon foam either before or afterexpansion to enhance specific mechanical properties such as fracturestrain, fracture toughness and impact resistance. For example,particles, whiskers, fibers, plates, etc. of appropriate carbonaceous orceramic composition can be incorporated into the carbon foam to enhanceits mechanical properties.

The cooling step in the expansion process results in some relativelyminimal shrinkage on the order of less than about 5% and generally inthe range of from about 2% to about 3%. This shrinkage must be accountedfor in the production of ear net shape carbon foams of specificdimensions and is readily determinable through trial and error with theparticular coal starting material being used. The shrinkage may befurther minimized by the addition of some inert solid material such ascoke particles, ceramic particles, ground waste from the coal expansionprocess etc. as is common practice in ceramic fabrication.

Carbonization is conventionally performed by heating the coal-based,carbon foam prepared as just described, under an appropriate inert gasat a heat-up rate of less than about 5° C. per minute to a temperatureof between about 800° C. and about 1200° C. and soaking for about 1 houror less. Appropriate inert gases are those described above that aretolerant of these high temperatures. The inert atmosphere is supplied ata pressure of from about 0 psi up to a few atmospheres. Thecarbonization process serves to remove all of the non-carbon elementspresent in the preform or product such as sulfur, oxygen, hydrogen, etc.

Graphitization, commonly involves heating the carbon foam either beforeor after carbonization at heat-up rate of less than about 10° C. perminute, preferably from about 1° C. to about 5° C. per minute, to atemperature of between about 1700° C. and about 3000° C. in anatmosphere of helium or argon and soaking for a period of less thanabout one hour. Again, the inert gas may be supplied at a pressureranging from about 0 psi up to a few atmospheres.

Coals suitable for use in the processes described herein are primarilybituminous coals exhibiting a “swell index” as determined by ASTMstandards DD5515-97, “Standard Test Method for the Determination ofSwelling Properties of Bituminous Coal” and D720-91 “Standard TestMethod for Free Swelling Index of Coal” of between about 3 and about 9and preferably about 4. Best results are achieved in terms of adequatecell generation to obtain coal-based cellular materials of the properdensities when the bituminous coal demonstrates a Gieseler plasticityvalue commonly characterized as high, i.e. above about 500 DDPM. Suchvalues are determined in accordance with ASTM standard D-2639.Agglomerating bituminous coals, i.e. those containing from about 10 toabout 32% by weight volatiles are specifically preferred.

The carbon foams resulting from processing in accordance with theforegoing procedures are optimally useful as fuel cell electrodes asdescribed below.

As already alluded to, the coal-based, carbon foams can be produced inany solid geometric shape. Such production is possible using any numberof modified conventional processing techniques such as extrusion,injection molding, etc. In each of such instances, the process must, ofcourse, be modified to accommodate the processing characteristics of thestarting material coal. For example, in extruding such products, asdescribed below, the coal powder starting material is fed by an augerinto an expansion chamber where it is expanded and from which it isextruded while still viscous. Upon exiting the extrusion die, thematerial is cooled to provide a solid shape of the desired andprecalculated dimensions. To improve the efficiency, i.e., cycle time ofthe process, the input material can be preheated to a temperature belowthe expansion point, e.g., below about 300° C., fed into the augerchamber where additional heat is imparted to the powder with finalheating being achieved just before extrusion through the die.

Similar relatively minor process modifications can be envisioned tofabricate the carbon foams of the present invention in injectionmolding, casting and other similar conventional material fabricationprocesses.

Foams prepared as just described are very different than their naturalgraphite, coke, and mesophase microbead cousins—and even competitivecarbon foams—in several important ways. Unlike natural graphite, suchfoams are harder and more durable owing to that fact that they arecomprised of finer crystallites that are randomly oriented. Thus, theyare not likely to degrade structurally or exfoliate during service, asdoes natural graphite, even in the presence of aggressive electrolytes,such as propylene carbonate. Coke products are similar in that they areproduced from petroleum or coal feedstocks. However, cokes have lowcapacity and can contain significant amounts of impurities, like sulfur,hydrogen, and ash, which interact deleteriously with metal ions andelectrolyte to create unwanted half-cell reactions. While they can beporous and hard like the porous coal-based foams described herein, cokestructure and uniformity is not controlled—and cannot be designed, ascan that of the instant materials. Cleaning the raw coal before foaming,designing the cellular structure during the foaming process, and heattreating the foam to a higher temperature than coke to remove sulfur andhydrogen, provides coal-based foams that offer significant anodeperformance benefits over coal at nearly as low a price. High qualitypitch-based microbeads and foams can offer additional performancebenefits and design potential, but these materials cannot approach thelow cost of foamed coal residues. Thus, if plotted on a line chart, theperformance and cost of these potential anode materials might berepresented as

performance

(poor) coke coal-based foam mesophase microbeads (excellent)

cost

(low)coke coal-based foam mesophase microbeads (high)

The invention will be better understood when considered in light of thefollowing, non-limiting examples of its implementation.

EXAMPLES Example 1

Coal-based, carbon foam fuel cell electrodes were produced from fourdifferent coal precursors and heat-treated them at several differentgraphitization temperatures for initial evaluation in fuel cellapplications. An additional foam article, produced from a high-purity(and expensive) synthetic pitch, and heat treated to a very hightemperature, was also included for comparison. A description of the fourcoals and one pitch employed in this study is presented in Table 1.

TABLE 1 Descriptions of coals included in this evaluation ID Coal IDSeam Comments A 610 Elkhorn Pike County Stoker, High-Vol. B 729 CoalburgWayne County Washed, Deep Mine, High-Vol C 900 Powellton PowelltonCountry High-Volatile (POW) D 800 Sewell McDowell County Stoker,Low-Vol. M N/A N/A Mitsubishi ARA24 Resin (synthetic mesophase fromnaphthalene)

The four coals were characterized by (1) constituent analysis, (2)ultimate analysis, (3) free-swell index (in accordance with ASTM D720),and (4) Gieseler plasticity (in accordance with ASTM D2639). These dataare summarized in Table 2.

TABLE 2 Summary of analytical data for 610, 729, POW, and 800 coalsMaterial => 610 729 POW 800 Constituent Analysis % Total Moisture 2.504.79 1.36 4.48 % Ash (A/R) 9.00 9.19 7.35 7.50 % Ash (D/B) 9.23 9.657.45 7.85 % Sulfur (A/R) 1.23 0.84 0.89 0.64 % Sulfur (D/B) 1.26 0.880.90 0.67 BTU/lb (A/R) 13,226 12,757 14,103 13,851 BTU/lb (D/B) 13,56513,399 14,298 14,501 MAFBTU 14,945 14,830 15,448 15,736 Free SwellingIndex 4 4 4 9 Ultimate Analysis % Carbon (D/B) 71.03 74.99 84.09 82.61 %Hydrogen (D/B) 5.24 5.25 5.30 4.40 % Nitrogen (D/B) 1.38 1.35 2.44 1.07% Ash (D/B) 9.23 9.65 0.50 7.85 % Sulfur (D/B) 1.26 0.88 0.82 0.67 %Oxygen (D/B) 11.86 7.88 6.85 3.40 Gieseler Plasticity Max Fluidity(ddpm) 2,080 227 4,618 132 Max Fluidity Temp (C.) 430 439 456 474Initial Softening Temp (C.) 390 397 395 437 Solidification Temp (C.) 458460 495 496 Plastic Range (C.) 68 63 100 59

Example 2 Foaming

Carbon foam test articles were fabricated by foaming (thermallydecomposing under controlled temperature and pressure conditions toproduce cellular material of uniform density) and calcining (heattreating in an inert atmosphere to remove aliphatic material from thecarbon structure) bituminous coal powders. Foaming took place inreactors manufactured by Parr Instruments (Moline, Ill.) at temperaturesbetween 425 and 550° C. and nitrogen pressures between 0 (ambient) and500 psi. Two identical Model 4570 high-pressure reactors, with 600° C.temperature and 3000 psi pressure service capabilities, were used inthis evaluation.

Two hundred grams of material were used to produce each article, and thearticles were formed in a 6061 aluminum can 5.0-inches in diameter by6.0-inches in height. The process employed for foam article productionwas as follows.

-   -   Load reactor with container filled with 200 grams of −60 mesh        coal powder,    -   Seal reactor by tightening sixteen (16) set screws in star        pattern to 30 ft-lbs in four cycles (one at 10 ft-lbs, one at 20        ft-lbs, and two at 30 ft-lbs) using a torque wrench,    -   pressurize system to at least 100 psi nitrogen,    -   vent reactor to 0 psi,    -   pressurize reactor and ballast tanks to the desired operating        pressure for the experiment (0 to 500 psi),    -   seal-off gas supply tanks, retaining connection between reactor        and ballast tanks to moderate pressure excursions during        operation,    -   start heating profile, which includes five steps    -   heat at 2.5° C. per minute from ambient temperature to 325° C.,    -   soak at 325° C. for one hour,    -   heat at 2.0° C. per minute from 325° C. to the ultimate        temperature (between 450 and 550° C., dependent upon the maximum        fluidity temperature of the coal),    -   soak at the ultimate temperature for seven hours, and    -   cool from the ultimate temperature to 25° C. at an uncontrolled        rate,    -   vent the reactor when temperature was between 50 and 100° C.,        and    -   loosen bolts by reversing the tensioning procedure at 30+ ft-lbs        torque and remove the containers.

Example 3 Calcining

All articles were calcined to 1050° C. by loading foam articles into a304 stainless steel can and surrounding the articles with granulatedcoke breeze (Loresco Earth Backfill). The coke breeze served twopurposes: (1) it increased thermal mass of the system to minimizetemperature excursions and influence of heating element cycling, and (2)it scavenged oxygen from the furnace atmosphere. The calcining cycle wasas follows.

-   -   heating from ambient temperature to 1050° C. at a heating rate        of 1.0° C. per minute,    -   remaining at 1050° C. for two to three hours, and    -   cooling at an uncontrolled rate (furnace power turned off).

A summary of the mass losses, shrinkages, densities, and compressivestrength/modulus properties for the different coal precursors at theas-foamed (“green”) and calcined stages is presented in Table 3.

TABLE 3 Summary of foaming and calcining data for coal precursorsPrecursor Summary Parameter 610 729 POW 800 average maximum minimumFoaming Temperature (C.) 450 500 500 550 500 450 550 Foaming Pressure(psi) 500 500 500 500 500 500 500 Foaming Mass Loss (%) 17.12% 20.39%13.90% 10.16% 15.39% 10.16% 20.39% “Green” Foam Bulk Density (g/cc)0.3740 0.3814 0.3857 0.3787 0.3800 0.3740 0.3857 “Green” Foam Strength(psi) 308 460 752 1,009 632 308 1,009 “Green” Foam Modulus (ksi) 22 3239 50 36 22 50 Calcining Temperature (C.) 1050 1050 1050 1050 1050 10501050 Calcining Mass Loss (%) 18.12% 13.49% 11.62% 9.95% 13.30% 9.95%18.12% Total Mass Loss (%) 32.14% 31.13% 23.91% 19.10% 26.57% 19.10%32.14% Calcined Foam Bulk Density (g/cc) 0.4569 0.4887 0.4974 0.47070.4784 0.4569 0.4974 Calcining Shrinkage (%, linear) 11.97% 10.85%11.38% 9.43% 10.91% 9.43% 11.97% Calcined Foam Strength (psi) 1,3491,512 2,393 2,431 1,921 1,349 2,431 Calcined Foam Modulus (ksi) 103 77126 89 99 77 126

Example 4 Graphitization

Calcined foam articles, nominally 5.0-inches in diameter and less than3.0-inches in thickness, were quartered so that they would fit in thechamber of a Thermal Technologies “Astro” graphite resistance furnacefor graphitization. A sample of each of the coal foams was stacked inthe vertical cylindrical hot zone (approximately 30-inches diameter by6.0-inches length) of the furnace, atop a graphite hearth and separatedby flexible graphite foils. The graphitization process was performed asfollows:

-   -   carbon foam samples loaded into hot zone and furnace sealed by        means of three knurled screws,    -   furnace shell water cooling system started,    -   furnace evacuated by roughing pump and then pressurized with        helium to 2 to 5 psi gage pressure (this cycle is repeated three        times),    -   furnace filled to 2 to 5 psi helium,    -   heating profile started, with heating rate set at 20° C. per        minute to an ultimate temperature selected from 1800, 2000,        2200, 2400, and 2600° C.,    -   furnace held at ultimate temperature for 30 minutes,    -   furnace turned off and allowed to cool without a controlled        rate, and    -   furnace pressure vented and samples removed.

Specimens at least 2.0-cm in diameter and 1.0-cm in thickness from eachprecursor/heat treatment combination were subjected to voltammetry andother characterization. Remnant specimens were subjected to (1) X-raydiffraction analysis, (2) mechanical testing, and (3) physical testingto determine the effects of high temperature treatment on thesedifferent materials. These data are summarized below.

Example 5 Carbon Foam Mechanical and Physical Properties

Sample Preparation and Identification

The four coals for which the structural changes that occur duringgraphitization were studied were labeled as follows:

-   -   A=610 coal,    -   B=729 coal,    -   C=POW coal, and    -   D=800 coal.        For each coal, samples heat-treated to different ultimate        temperatures are noted as    -   1=1600° C.,    -   2=1800° C.,    -   3=2000° C.,    -   4=2200° C.,    -   5=2400° C., and    -   6=2600° C.,        with the exception of lot D (800 coal), for which less calcined        foam was available. For lot D, then, the samples are noted as    -   1=1600° C.,    -   2=2000° C.,    -   3=2400° C., and    -   4=2600° C.

Thus, sample A1 was 610 coal foam graphitized to 1600° C. and sample C5was POW coal foam graphitized to 2400° C. Foam made from MitsubishiARA24 resin graphitized to 2600° C. was labeled Lot M.

X-Ray Diffraction

Wide angle X-ray diffraction (copper K_(α) radiation) was employed tostudy the crystal structure of the graphitized foams. The raw intensityversus Bragg angle scans were studied and analyzed according to commonlyaccepted methods.

The interplanar spacing, or spacing between the (00,2) crystal planesfor graphite, is an important descriptor of crystal order. This spacingis 3.44 Angstroms for “turbostratic,” or disordered graphite with random(00,2) plane orientation, and 3.354 Angstroms for perfectly orderedgraphite, with an ABAB stacking sequence. Values of d(00,2) valueshigher than that of disordered graphite are possible—with strain,heteroatom inclusion, etc.

Crystallite size can also be calculated from the breadth of multipleorders of the same reflections (peaks), after separation of instrumentaland strain broadening terms, and application of Scherrer's Law. Thedetermination of two crystallite dimensions is possible. The first isLc, the stack height, which is the crystallite size in the directionnormal to the (00,2) planes, and is determined from the breadths of(00,2), (00,4), (00,6), etc. types of reflections. This indicates theheight of a stack of (00,2) planes ordered in the ABAB stackingsequence. La, the coherence length, is more difficult to define, but isrelated to the diameter or breadth of graphite planes. The planes areactually very large in dimension, but La indicates distances betweenwrinkles, dislocations, and other such features that scatter X-rays,rather than the breadth of a discrete, disk-like solid. La correlateswell with transport properties, like thermal and electricalconductivity, as the conducting phonons and electrons are scatter atthese crystal imperfections in similar fashion as do X-rays—though notat exactly the same degree. La, then, can be correlated with themean-free-paths of phonons, for example, in the prediction of thermalconductivity.

Table 4 summarizes the crystal parameters calculated from the X-rayscans, and FIGS. 11 through 14 illustrate the dependence of some ofthese parameters on heat treatment temperature. The following tables andFIGS. 9 through 12 summarize these data.

TABLE 4 Crystal properties of coal foams after graphitization totemperatures between 1600 and 2600° C. d(002) Stack Height CoherenceLength La/Lc Raw Material Heat Treatment Angstroms g (Angstroms)(Angstroms) Ratio A 1600 3.4812 −0.4790 41.3 31.2 0.76 610 1800 3.4549−0.1727 49.6 39.5 0.80 2000 3.4368 0.0375 71.9 43.1 0.60 2200 3.43350.0757 73.4 44.0 0.60 2400 3.4365 0.0403 70.8 129.5 1.83 2600 3.43380.0724 72.1 647.4 8.98 B 1600 3.4779 −0.4411 41.0 16.8 0.41 729 18003.4612 −0.2470 53.1 18.2 0.34 2000 3.4424 −0.0275 81.5 38.1 0.47 22003.4315 0.0984 94.9 82.0 0.86 2400 3.4389 0.0125 91.1 33.9 0.37 26003.4312 0.1019 95.5 100.0 1.05 C 1600 3.4629 −0.2667 51.7 35.0 0.68 POW1800 3.4485 −0.0993 65.1 12.4 0.19 2000 3.4360 0.0460 117.5 33.7 0.292200 3.4278 0.1416 141.2 202.6 1.44 2400 3.4101 0.3479 150.5 39.7 0.262600 3.4238 0.1878 155.1 46.4 0.30 D 1600 3.4550 −0.1739 56.0 27.1 0.48800 1800 2000 3.4340 0.0693 130.9 67.1 0.51 2200 2400 3.4315 0.0988187.9 315.5 1.68 2600 3.4309 0.1063 184.7 311.5 1.69 MIT 2600 3.38040.6926 338.5 556.1 1.64Mechanical and Physical Testing

The mechanical and physical properties, including (1) bulk density, (2)solid (or true) density, (3) mass and dimensional change duringgraphitization (relative to calcined mass and dimension), (4) porosity,(5) compressive strength, and (6) compressive modulus, were determinedto supplement the electrochemical performance data. Specific methods aredescribed in the following subsections, and data are presented in theTable 5 and FIGS. 13 through 17.

Mass Loss

Mass losses during foaming, calcining, and graphitization werequantified for correlation with the volatile matter content, ashcontent, and content/forms of sulfur of the different coal precursors.For foaming, the mass of the coal powder was quantified usingconventional techniques. For calcining and graphitization, the mass ofthe foam articles was weighed using the same Ohaus balance before andafter heat treatment.

Density

Bulk density was determined by measurement using a vernier caliper,which also allowed quantification of dimensional change, and weighing.Solid, or “true,” density was determined by helium pycnometry(Micromeritics AccuPyc 1330).

Compressive Strength/Modulus

Compression tests were conducted by test method ASTM C365. It was foundthat compressive strength data were essentially independent of samplegeometry down to sample loading surfaces as small as 0.75-inch square.The platen displacement rate was also studied, and a rate of 0.05 inchesper minute was settled on as the standard. Uniform load transfer wasaccomplished by simply attaching 3M packaging tape to the loaded facesof the foam.

TABLE 5 Mechanical and physical properties of coal foams graphitizedover a range of temperatures Solid Raw Heat Bulk Density COV Mass ChangeDimension Change Density E, compressive S, compressive MaterialTreatment (g/cc) on Density (%, rel. to 1050) (%, rel. to 1050) (g/cc)Porosity K (psi) (psi) 610 1050 0.3758 0.0033 0.00% 0.00% 1.9033 0.7943.85 56,200 1,158 1600 0.3755 0.0002 5.98% 2.25% 1.9156 0.796 3.9052,900 1,171 1800 0.3629 0.0034 9.55% 2.65% 1.9188 0.801 4.03 59,3001,134 2000 0.3682 0.0045 12.03% 3.30% 1.9355 0.800 4.00 53,300   9862200 0.3526 0.0018 17.04% 4.13% 1.9065 0.807 4.18 44,100   873 24000.3714 0.0076 13.95% 4.20% 1.9738 0.810 4.26 23,900   900 2600 0.36290.0011 16.22% 4.07% 1.9495 0.811 4.29 40,600   877 POW 1050 0.39730.0004 0.00% 0.00% 1.9208 0.793 3.83 87,800 1,425 1600 0.3897 0.00662.86% 2.39% 2.0266 0.810 4.26 57,500   829 1800 0.3891 0.0033 5.06%2.41% 2.0353 0.810 4.26 54,000   958 2000 0.3707 0.0016 8.86% 2.35%2.0691 0.820 4.56 36,700   802 2200 0.3658 0.0029 11.48% 3.11% 2.06480.822 4.62 48,900   710 2400 0.3787 0.0010 13.21% 3.86% 2.0369 0.8144.38 32,700   839 2600 0.3945 0.0060 10.47% 3.78% 2.0647 0.807 4.1850,200   914 729 1050 0.5588 0.0065 0.00% 0.00% 1.9059 0.709 2.44152,000  3,097 1600 0.5093 0.0023 9.98% 2.82% 1.9891 0.745 2.92 87,3002,729 1800 0.5045 0.0033 13.62% 3.56% 2.0062 0.747 2.95 109,000  2,5712000 0.4990 0.0065 16.81% 4.10% 1.9923 0.752 3.03 108,000  1,865 22000.4886 0.0078 19.64% 4.63% 1.9731 0.755 3.08 97,100 1,646 2400 0.50480.0036 19.95% 4.63% 1.9878 0.747 2.95 78,900 1,688 2600 0.4694 0.000020.18% 4.34% 1.9436 0.758 3.13 75,800 1,480 800 1050 0.4905 0.0000 0.00%0.00% 1.9854 0.753 3.05 132,000  2,384 1600 0.4836 0.0004 3.50% 1.58%2.0950 0.769 3.33 85,400 1,588 1800 2000 0.4568 0.0093 8.27% 1.65%2.1228 0.782 3.59 75,100 1,089 2200 2400 0.4624 0.0066 8.99% 1.84%2.1345 0.781 3.57 54,000 1,026 2600 0.4581 0.0015 8.82% 1.45% 2.12520.784 3.63 47,300   986

Example 6 Carbon Foam Electrical Properties

Cyclic Voltammetry

The performance of these foams were studied using a cyclic voltammetry(CV) cell and ferricyanide (Fe²⁺+Fe³⁺ red-ox couple) testing in KClsolution. CV test electrodes were prepared from the carbon foam articlesfabricated as just described and CV scans performed to generateelectrode-specific voltammograms, post-CV analysis of the used carbonfoam electrode was also performed. Electrode performance was examined ina 5 M KCl solution containing ferricyanide (a Fe²⁺/Fe³⁺ redox couple)and compared to baseline electrode operation in an iron-free KClsolution. With these half-cell reactions, iron is being reduced at thecarbon electrode, with the reference electrode being Calumel (Ag/AgCl).In these tests, a scan rate of 5-50 mV/sec was used to generate aclosed-loop voltammogram, which was be compared to that of glassy carbon(see FIG. 18) as a benchmark.

Basis of Cyclic Voltammetry

Cyclic voltammetry consists of cycling the potential of a stationaryelectrode immersed in a quiescent solution and measuring the resultingcurrent. The excitation is a linear potential scan with a triangularwaveform. In this study, the scan rate was varied between 5 and 500mV/sec, depending upon the performance of the individual specimen. Thetriangular potential excitation signal sweeps the potential of theworking electrode back and forth between two designated values calledthe “switching potentials.” A complete voltammogram includes the forwardand reverse sweep, and displays the cathodic (reduction) and anodic(oxidation) waveforms. The current at the working electrode is measuredunder diffusion-controlled, mass transfer conditions.

A schematic voltammogram is presented in FIG. 19. The scan begins at apotential of 0.0V (point a) and initial scan is in the negativedirection. When the potential becomes sufficiently negative (in thisexample, around −0.6V), reduction of electroactive species at theelectrode surface is initiated and cathodic current begins to flow(point b). The cathodic current increases rapidly until the surfaceconcentration of oxidant approaches zero and the current, nowdiffusion-limited, peaks at point c. As the potential becomes morenegative, the current decays in proportion to t^(−1/2), according to theCottrell equation, as the solution surrounding the electrode is depletedof oxidant (which is converted to the reduced state). A fine rise incurrent (at point d) results from discharge of the electrolyte solution.At the switching potential (here −0.9V), the scan is reversed to thepositive direction. However, the potential remains sufficiently negativeto continue reduction of the oxidant, and the cathodic current continuedfor part of the reverse (positive) scan. Eventually, the potentialbecomes sufficiently positive to oxidize the reductant that had beenaccumulating near the electrode surface. Anodic current begins to flowand counterbalance the cathodic current. Then, in similar fashion asdescribed for the cathodic process, anodic current increases rapidlyuntil the concentration of reductant at the electrode surface approacheszero (point f), and peaks. The anodic current then decays as thesolution surrounding the electrode is depleted of reductant. Whenreturned to the starting potential, the reduced material is stillpresent at the electrode and oxidizes back to the initial form of thecouple.

The important parameters that are recorded from a voltammogram include:

anodic peak current, (i_(p))_(a),

cathodic peak current, (i_(p))_(c),

anodic peak potential, (E_(p))_(a),

cathodic peak potential, (E_(p))_(c), and

half-peak potential, (E_(p/2))_(c).

The peak current for the oxidant, assuming initial cathodic scan, isgiven by

${i_{p} = {n^{\frac{3}{2}}{F^{\frac{3}{2}}\left( \frac{{\pi\vartheta}\; D_{ox}}{RT} \right)}^{\frac{1}{2}}A\; C_{ox}{\chi\left( {\sigma\; t} \right)}}},$where

-   -   i_(p) is in amperes,    -   n is the number of electrons transferred in the electrode        reaction,    -   R is in JK⁻¹ mol⁻¹,    -   T is in Kelvin,    -   A is area, in cm²,    -   D is the diffusion coefficient, in cm²/sec,    -   C is the concentration, in mol/cm³,    -   υ is the scan rate, in V/sec, and    -   χ(σt) is a tabulated function, whose value is 0.446 for a        simple, diffusion-controlled electron transfer reaction.

For a reversible wave, E_(p) is independent of scan rate, and i_(p) isproportional to v^(1/2). The number of electrons transferred, n, can bedetermine from the separation in cathodic and anodic potential peaks, by

${\left( E_{p} \right)_{a} - \left( E_{p} \right)_{c}} = {\frac{0.057}{n}.}$

The formal potential for the reversible couple is centered between thecathodic and anodic peak potentials. Irreversibility of a wave isindicated by the lack of a reverse peak. Reversibility can also beevaluated by plotting (i_(p))_(c) or (i_(p))_(c) versus the square rootof the scan velocity. Such plots should be linear with intercepts at theorigin.

Voltammogram Analysis

Analysis of the carbon foam voltammograms allowed checking for Nernstianbehavior, adequate current density, and stability of the voltammogramshape (i.e., scan rate pattern) over an extended test period. The latterdemonstrated whether the carbon foam adsorbs ions from solution,resulting in contamination that would inhibit current flow. A positiveresult from these tests would is a high current density (relative tothat of a glassy carbon electrode) and a scan rate pattern that could bemaintained over several CV cycles with minimal current degradation. Apost-CV analysis of the carbon foam electrode with energy dispersiveX-ray spectrometer analysis (EDS) tested for electrode contamination byoxygen, iron, or other impurities that might degrade performance. Suchanalysis indicated minimal such contamination.

A key to the CV screening tests was to prepare the working coal-based,carbon foam electrodes (and the glassy carbon reference) to be bothcompatible with the voltammeter apparatus and representative of the typeof service it would experience in an actual fuel cell. The recommendedprocedure is to machine the carbon foam as a monolithic insert to theworking electrode of the CV apparatus and epoxy it in place. A mercurydroplet was used to assure good conductivity between the carbon foammaterial and the copper conductor in the electrode connector. The foam'spores could then be filled with Nugel™ (or mineral oil) to eliminatemass transfer effects in the pore structure during the CV screeningtests. To further analyze for mass transfer processes in the foams, andtheir relationship to foam cell structure, the mineral oil was removedand CV tests repeated. A summary of cyclic voltammetry results for thedifferent precursor/heat treatment combinations considered as part ofthe evaluation study is presented in Table 6.

TABLE 7 Summary of electrode performance in ferricyanide system SolutionKCl w/K₃Fe(CN)₆ Scan Rate 5 25 50 Schedule Sample I_(p) (A) E_(pa) (mV)E_(pc) (mV) I_(p) (A) E_(pa) (mV) E_(pc) (mV) I_(p) (A) E_(pa) (mV)E_(pc) (mV) 0 Glassy C 1.872E−05 233.5 3.819E−05 222.6 6.557E−05 218.1 1A1-1600 1.349E−04 311.8 247.1 3.851E−04 342.2 228.1 5.159E−04 355.2212.9 1 A2-1800 5.206E−05 332.9 193.9 1.044E−04 N/A 152.1 1.298E−04 N/A114.1 1 A3-2000 9.228E−05 337.1 220.5 2.087E−04 405.2 178.7 2.743E−04N/A 152.1 1 A4-2200 1.143E−04 291.4 197.7 2.201E−04 326.2 133.12.399E−04 341.3 95.1 2 A5-2400 1.066E−04 270.1 178.7 2.402E−04 310.4129.3 3.143E−04 343.1 102.7 2 A6-2600 7.683E−05 247.5 144.5 1.774E−04302.1 83.7 2.377E−04 344.3 41.8 2 A7-1050 5.175E−05 288.4 201.51.025E−04 317.9 155.9 1.188E−04 321.6 129.3 2 B1-1600 8.889E−05 304.2197.7 2.106E−04 342.2 171.1 2.852E−04 357.4 155.9 3 B2-1800 8.889E−05302.9 220.5 2.540E−04 349.4 174.9 3.513E−04 385.6 148.3 3 B3-20001.175E−04 287.3 197.7 3.270E−04 356.8 136.9 4.444E−04 393.3 95.1 3B4-2200 1.586E−01 284.8 205.3 4.190E−04 333.0 159.7 5.503E−04 352.8133.1 3 B5-2400 8.222E−05 269.3 186.3 1.902E−04 311.9 114.1 2.584E−04343.7 76.0 4 B6-2600 1.109E−04 325.1 220.5 2.040E−04 375.3 193.92.188E−04 382.1 190.1 4 B7-1050 5.071E−05 303.9 205.8 1.136E−04 342.2154.6 1.502E−04 375.6 111.7 4 C1-1600 6.085E−05 312.8 199.0 1.048E−04338.5 128.1 1.062E−04 383.2 75.2 4 C2-1800 1.298E−05 301.8 223.32.250E−05 321.3 222.0 2.881E−05 324.2 215.4 5 C3-2000 6.667E−05 272.8159.4 1.567E−04 294.1 111.5 2.114E−01 311.4 97.5 5 C4-2200 9.947E−05300.0 197.3 1.862E−04 324.0 168.0 2.285E−04 331.9 161.8 5 C5-24009.841E−05 300.7 182.9 2.056E−04 344.6 11.0 2.690E−04 416.4 65.1 5C6-2600 5.757E−05 308.6 201.1 9.566E−05 346.6 179.3 1.082E−04 360.8153.6 6 C7-1050 1.031E−04 333.4 216.8 1.964E−04 444.9 133.1 1.206E−04361.2 182.5 6 D1-1600 4.804E−05 295.4 237.7 1.159E−04 319.4 211.81.604E−04 339.4 191.8 6 D2-2000 9.503E−05 268.3 180.6 2.382E−04 316.2103.8 2.502E−04 340.1 47.3 6 D3-2400 7.143E−05 297.8 222.2 1.468E−04339.2 203.7 1.786E−04 389.1 196.6 7 D4-2600 6.349E−05 308.7 242.81.548E−04 339.0 225.0 2.143E−04 363.0 209.3 7 MB-2700 5.778E−05 340.7175.4 1.045E−04 382.5 54.4 N/A N/A N/A 7 G80 1.871E−05 308.0 231.93.594E−04 389.5 174.0 5.074E−04 429.7 138.9 7 G100 N/A N/A N 2.646E−04399.2 159.7 3.439E−04 448.7 125.5 Solution KCl w/K₃ Fe(CN)₆ Scan Rate100 500 Schedule Sample I_(p) (A) E_(pa) (mV) E_(pc) (mV) I_(p) (A)E_(pa) (mV) E_(pc) (mV) 0 Glassy C 6.811E−05 209.3 1.344E−04 175.8 1A1-1600 6.984E−04 374.1 201.5 1.476E−03 N/A 152.1 1 A2-1800 3.108E−04N/A −448.7 N/A N/A N/A 1 A3-2000 3.526E−04 N/A 106.5 N/A N/A N/A 1A4-2200 2.783E−04 360.6 76.0 4.825E−04 N/A 38.0 2 A5-2400 4.127E−04387.3 76.0 9.312E−04 N/A −102.7 2 A6-2600 3.042E−04 N/A −7.6 N/A N/A N/A2 A7-1050 1.444E−04 332.8 106.5 2.635E−04 393.3 53.2 2 B1-1600 3.704E−04384.0 133.1 6.074E−04 494.3 22.8 3 B2-1800 4.413E−04 435.0 114.1 N/A N/AN/A 3 B3-2000 5.556E−04 428.4 68.4 N/A N/A N/A 3 B4-2200 7.238E−04 367.6102.7 N/A N/A N/A 3 B5-2400 3.394E−04 384.2 30.4 N/A N/A N/A 4 B6-26002.434E−04 402.7 182.5 4.683E−04 497.3 114.1 4 B7-1050 1.936E−04 428.751.8 2.730E−04 608.4 −121.7 4 C1-1600 1.240E−04 426.8 62.0 2.525E−04556.7 −35.7 4 C2-1800 3.631E−05 335.1 205.4 7.143E−05 356.7 183.0 5C3-2000 2.895E−04 335.4 75.7 6.000E−04 484.4 −26.0 5 C4-2200 2.442E−04343.3 161.8 6.349E−04 398.1 118.9 5 C5-2400 3.563E−04 514.3 13.27.452E−04 N/A −232.3 5 C6-2600 N/A N/A N/A N/A N/A N/A 6 C7-10501.429E−04 391.8 167.3 1.772E−04 452.5 79.8 6 D1-1600 2.142E−04 367.3166.3 N/A N/A N/A 6 D2-2000 3.244E−04 399.4 −15.6 N/A N/A N/A 6 D3-24002.143E−04 401.9 180.9 N/A N/A N/A 7 D4-2600 2.857E−04 396.5 188.4 N/AN/A N/A 7 MB-2700 N/A N/A N/A N/A N/A N/A 7 G80 5.291E−04 494.3 95.1 N/AN/A N/A 7 G100 4.497E−04 551.3 63.7 N/A N/A N/A Note: A = Foam producedfrom an extract of 610 (high-volatile bituminous) coal B = Foam producedfrom an extract of 729 (high-volatile bituminous) coal C = Foam producedfrom an extract of Powellton (high-volatile bituminous) coal D = Foamproduced from an extract of 800 (high-volatile bituminous) coal MB =“standard” foam produced from Mitsubishi ARA24 synthetic resin(mesophase pitch derived from napthalane) Number after “—” are graphitetemperature (° C.)Cyclic Voltammetry Scans

Voltammograms for example materials are presented in the followingfigures, including:

-   -   FIG. 20: glassy carbon (control standard),    -   FIG. 21: Mitsubishi ARA24 synthetically-prepared mesophase pitch        foam (labeled M or MB),    -   FIG. 22: A1 (610 coal-based foam, heat treated to 1600° C.),    -   FIG. 23: B4 (729 coal-based foam, heat treated to 2200° C.),    -   FIG. 24: C4 (Powellton coal-based foam, heat treated to 2200°        C.),    -   FIG. 25: C7 (Powellton coal-based foam, heat treated to 1050°        C.),    -   FIG. 26: C1 (Powellton coal-based foam, heat treated to 1600°        C.), and    -   FIG. 27: C6 (Powellton coal-based foam, heat-treated to 2600°        C.).

Examined together, voltammograms from samples C4, then C7, C1 and C6(increasing in heat treatment temperature), illustrate the effects ofcombined graphitic ordering and impurity removal on performance.

From these evaluations, it can be seen that very well developed,Nernstian waveforms are developed in the case of B4 and C4 samples.These materials also offer very good current density, higher than thatof the glassy carbon electrode material. When only the C-series(Powellton coal) samples are considered at low scan rate (5 mV/sec), itcan be seen that the current density reaches a peak for the 2200° C.heat-treated sample (see Table 8). Lower heat treatment temperatures mayproduce too little graphitic ordering. At higher temperatures, thecrystal ordering—for this high-sulfur coal—is destroyed by theliberation of thiophenic sulfur as CS₂. Unless extracted prior tofoaming to remove essentially all of the aromatic sulfur, all such coal-or petroleum-based materials will exhibit similar behavior. That is, atemperature will exist that will provide optimum current density. Suchan effect would not be expected of the clean, synthetically preparedpitches, like Mitsubishi ARA24.

Example C-Series Samples

TABLE 7 C-series foam performance at low potential scan rate HeatTreatment I_(p) (A) @ E_(pa) (Mv) @ E_(pc) (mV) @ Sample (° C.) 5 mv/sec5 mV/sec mV/sec C7 1050 1.031 × 10⁻⁴ 333.4 216.8 C1 1600 6.085 × 10⁻⁵312.8 199.0 C2 1800 1.298 × 10⁻⁵ 301.8 223.3 C3 2000 6.667 × 10⁻⁵ 272.8159.4 C4 2200 9.947 × 10⁻⁵ 300.0 197.3 C5 2400 9.841 × 10⁻⁵ 300.7 182.9C6 2600 5.757 × 10⁻⁵ 308.6 201.1Performance at Varying Electrolyte Concentrations

The high performance foams (e.g., MB, B4, and C4, as well as the glassycarbon control) also were examined by recording the same voltammogram ata range of KCl concentrations. In all cases, the voltammograms were verystable with concentration, with the cathodic and anodic peaks separatingslightly with increasing concentration. MB and samples B4 and C4 alldemonstrated higher current density than the glassy carbon electrode,with sample B4 most closely approximating the performance of thesynthetic pitch-based foam.

Reversibility of the Couple

The reversibilities of the waveforms for (1) glassy carbon, (2)synthetic pitch-based foam, (3) sample B4, and (4) sample C4, and theirstability versus KCl concentration, were examined over a narrowpotential scan rate range. With the exception of sample C4, thewaveforms were all reversible in that the plot of peak current (I_(p))versus square root of applied potential (V^(1/2)) is linear.

Activation of the foams described herein with nickel or intercalationwith dichromate and other similar suitable electrode activators isclearly possible and foams that have been so activated are clearlycontemplated as being included within the scope of the appended claims.

Thus, the feasibility of employing low-cost bituminous coal-based carbonfoams as electrodes for fuel cells has been demonstrated. Foamsrepresenting a wide range of precursor compositions, foam cell sizes,and heat treatment temperatures have all produced satisfactoryelectrodes for such applications. The following have thus beendemonstrated for the use of these coal-base carbon foams as fuel cellelectrodes:

-   -   Nernstian behavior for the ferrosene redox couplet in KCl        solution,    -   improved electrode current density for carbon foams as compared        to conventional glassy carbons,    -   higher “active” surface area, by 1-2 orders of magnitude, than        glassy carbons,    -   significant mass transfer effects (indicated by CV hysteresis,        and intentionally not masked by the inclusion of fillers),    -   stability of voltammogram shape at different scan rates and KCl        concentration and over extended cycling periods,    -   that the carbon foam electrode does not suffer contamination and        concomitant degraded current flow,    -   inertness of carbon foams to a number of chemicals, including        KCl, phosphoric acid, methanol, propylene carbonate, etc., and    -   the low “active” surface area of current carbon foams (typically        within the narrow range of 0.1 to 2.0 m²/g).

Of the candidate precursor materials and heat treatment conditionsdescribed hereinabove, the foam based on synthetically-prepared pitch(“M”) offered the highest current density and quality of voltammogramshape. Samples B4 and C4 were, however, next in performance. Theseprecursors (729 and Powellton coals) are less than 1/100th the cost ofthe synthetic pitch, making these materials more attractive given theirsignificant cost advantages and relatively similar performance.

While the coal-based, carbon foams described herein have been sodescribed primarily in the context of their use in fuel cells thatinclude a polymer electrolyte membrane, it will be apparent to theskilled artisan that they are similarly useful in fuel cells such asphosphoric acid fuel cells as well as electrodes for batteries.

In this latter context, certain batteries may be described asconcentration cells powered by the transfer of ions between twoelectrodes: a cathode and an anode of, for example, intercalatedgraphite. The use of carbonaceous materials (derived from carbonprecursors) as anode electrodes has several advantages includingreliability, safety, and increased cycle life. Present anode materialsinclude carbonaceous materials, such as natural graphite, cokes, MesoCarbon Micro Beads (MCMB), and non-graphitizable carbon forms (e.g.,glassy carbons) but not coal-based carbon foams of the types describedherein. However, many critical parameters, such as surface area andporosity, are difficult to predict and control for these prior artmaterials. Surface area and porosity are crucial for batteryapplications since the formation of decomposition products on thesurface of the carbon (mainly due to reactions with the electrolyte) maylead to a passivating layer.

It is clearly contemplated that the coal-based carbon foams describedhereinabove are useful as one or both of the electrodes of batterydevices and in fuel cell applications that do not necessarily involvethe use of a polymer electrolyte membrane.

As the invention has been described, it will be apparent to thoseskilled in the art that the same may be varied in many ways withoutdeparting from the spirit and scope of the invention. Any and all suchmodifications are intended to be included within the scope of theappended claims.

1. A fuel cell comprising: a polymer electrolyte membrane; an anode; acathode; and conductors for the supply of electrical current to anelectrical load, wherein at least one of said anode and said cathodecomprises a coal-based carbon foam having 7.45% (D/B) to 9.65% (D/B)ash, a density in the range of about 0.1 g/cm³ to about 0.8 g/cm³ and apore size below about 2000 μm.
 2. The fuel cell of claim 1, wherein saidcoal-based carbon foam is produced from particulate coal having adiameter less than about 1 mm.
 3. The fuel cell of claim 1, wherein saidcoal-based carbon foam has a compressive strength below about 6000 psi.4. The fuel cell of claim 1, wherein said coal-based carbon foam isprepared from bituminous coal.
 5. The fuel cell of claim 1, wherein saidcoal-based carbon foam exhibits a pore size below about 100μ.
 6. Thefuel cell of claim 1, wherein said coal-based carbon foam has beengraphitized at a temperature ranging from about 1600° C. to about 3000°C.
 7. The fuel cell of claim 1, wherein said coal-based carbon foam hasbeen graphitized at a temperature between about 1800° C. and about 2200°C.
 8. The fuel cell of claim 1, wherein said coal-based carbon foam isprepared by a process comprising the steps of: comminuting coal to asmall particle size to form a ground coal; placing said ground coal in amold; heating said ground coal in said mold under a non-oxidizingatmosphere to a temperature of ranging from about 300° C. to about 700°C. to form an electrode preform; and graphitizing said electrode preformat a temperature ranging from about 1600° C. to about 3000° C.
 9. Anelectrical cell for the generation or storage of electrical powerthrough an electrochemical reaction, the electrical cell comprising: ananode; a cathode; and conductors for the supply of electrical current toan electrical load, wherein at least one of said anode and said cathodecomprises a coal-based carbon foam having 7.45% (D/B) to 9.65% (D/B)ash, a density in the range of about 0.1 g/cm³ to about 0.8 g/cm³ and apore size below about 2000 μm.
 10. The electrical cell of claim 9,wherein said coal base carbon foam is produced from particulate coalhave a diameter less than about 1 mm.
 11. The electrical cell of claim9, wherein said coal-based carbon foam has a compressive strength belowabout 6000 psi.
 12. The electrical cell of claim 9, wherein saidcoal-based carbon foam exhibits a pore size below about 100 μm.
 13. Theelectrical cell of claim 9, wherein said coal-based carbon foam has beengraphitized at a temperature ranging from about 1600° C. to about 3000°C.
 14. The electrical cell of claim 9, wherein said coal-based carbonfoam has been graphitized at a temperature ranging from about 1800° C.to about 2200° C.
 15. The electrical cell of claim 9, wherein saidcoal-based carbon foam is produced from bituminous coal.
 16. Theelectrical cell of claim 9, wherein said carbon foam is prepared by aprocess comprising the steps of: comminuting coal to a small particlesize to form a ground coal; placing said ground coal in a mold; heatingsaid ground coal in said mold under a non-oxidizing atmosphere to atemperature of ranging from about 300° C. to about 700° C. to form anelectrode preform; and graphitizing said electrode preform at atemperature ranging from about 1600° C. to about 3000° C.